1. Field of the Invention
This invention pertains to an oil recovery method, and more specifically to a method for recovering viscous oil or viscous petroleum from subterranean deposits thereof including tar sand deposits. Still more specifically, this method employs steam and light hydrocarbons in the C.sub.3 to C.sub.12 range and specific injection-pressurization and frequent drawdown cycles, initiated soon after initiating steam injection.
2. Description of the Prior Art
There are known to exist throughout the world many subterranean petroleum-containing formations from which petroleum cannot be recovered by conventional means because the petroleum contained therein is so viscous that it is essentially immobile at formation temperature and pressure. The most extreme example of viscous petroleum-containing formations are the so called tar sand or oil sand deposits such as those located in the western portion of the United States and northern Alberta, Canada, and in Venezula. Other lesser deposits are known to exist in Europe and Asia.
Tar sands are frequently defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well with ordinary production methods. The petroleum contained in tar sand deposits are generally highly bituminous in character. The sand portion is a fine grain quartz sand coated with a layer of water with viscous bituminous petroleum occupying much of the void space around the water-wet sand grains. A small amount of gas is sometimes also present in the void spaces. The sand grains are packed to a void volume of about 35%, which corresponds to about 83% by weight sand. The balance of the material is bituminous petroleum and water. The sum of the bituminous petroleum and water is usually equal to about 17%, with the bituminous petroleum portion thereof varying from about 2% to about 16%.
The sand grains are tightly packed in the formation in tar sand deposits but are generally not consolidated. The API gravity of the bituminous petroleum ranges from about 5 to about 8, and the specific gravity at 60.degree. F. is from about 1.006 to about 1.027. The viscosity of bituminous petroleum found in tar sand deposits in the Alberta region is in the range of several million centipoise at formation temperature, which is usually about 40.degree. F.
Although some petroleum has been obtained from tar sand deposits by strip mining, this is possible only in relatively shallow deposits and over 90% of the known tar sand deposits are considered to be too deep for strip mining at the present time. In situ separation of the bituminous petroleum by a process applicable to deep subterranean formation through wells completed therein must be developed if significant amounts of the bituminous petroleum are to be recovered from the deposits which are too deep for strip mining purposes. The methods proposed in the literature to date include steam injection, in situ combustion, solvent flooding processes and steam-emulsification drive process.
Canadian Pat. No. 1,004,593 describes a promising oil recovery method once proposed for use in recovering viscous petroleum from the Peace River Oil Sand Deposits in Alberta, Canada, described in the July 3, 1974 Edition of the Daily Oil Bulletin. It comprises a steam injection-pressurization program wherein steam is injected for long periods of time while maintaining little or no production, sufficient to build the steam pressure in the formation to a value as high as 800 to 1100 pounds per square inch, followed by a prolonged soak period to effect maximum utilization of the thermal energy injected into the formation in the form of steam sufficient to reduce the viscosity of substantially all of the oil in the formation to a very low level, such that it will flow readily. Production is then initiated after the injection and soak cycle had been completed, and it is anticipated that several years will be required for completion of each injection period and soak cycle.
U.S. Pat. No. 3,155,160 describes a single well, push-pull steam only injection process employing alternating pressurization and production cycles to maintain pressure in the ever expanding cavity created adjacent the well by oil recovery.
Despite many proposed methods for recovering viscous petroleum from subterranean viscous petroleum-containing formations including the deep tar sand deposits, there has still been no commercially successful exploitation of deep deposits by in situ separation means up to the present time. In view of the fact there are enormous reserves in the form of viscous petroleum-containing deposits, (estimates of the Athabasca Tar Sand Deposits range upward to 700 billion barrel of petroleum) there is a substantial, unsatisfied need for an efficient, economical method for recovering viscous, bituminous petroleum from deep tar sand deposits.